The present invention relates to a wedge plate type light guiding plate for a front light.
To reduce the power consumption, some conventional display units have a lighting device. When there is sufficient outside light such as sunlight or room lighting, this type of display unit uses outside light. When the outside light is insufficient, the display unit uses the lighting device. Such display units include a reflective display unit with a front light. The front light of such a display unit has a light guiding plate. As the light guiding plate, there are a flat plate type and a wedge plate type.
FIG. 6 shows a reflective display unit 50 with a front light. The display unit 50 includes a light source 51, a light guiding plate 52, and a reflective liquid crystal panel 60. The liquid crystal panel 60 is located below the exit plane 53 of the light guiding plate 52. A number of grooves 57 having saw-tooth cross-section are formed on the reflection-exit 55. The light guiding plate 52 admits light through the incidence plane 54. The admitted light is reflected by the light admission plane 57a of each groove 57 and exits the light guiding plate 52 through the exit plane 53.
It is desirable that the brightness distribution of a reflective liquid crystal display unit be uniform. Specifically, light from the light source 51, which is reflected by the grooves 57 and guided to the reflective liquid crystal panel 60, preferably uniformly illuminate the panel 60.
Brightness uniformity is an index representing a brightness distribution. To compute the brightness uniformity, the values of brightness at specific points on the light guiding plate 52 are measured. The brightness uniformity is computed based on the maximum value and the minimum value of the measured brightness. Specifically, the brightness uniformity is computed in the following manner. First, the surface of the light guiding plate 52 is divided into sixteen sections by three parallel lines and another three lines perpendicular to the first three lines. The brightness at each of the nine intersection points L1 to L9 is measured. Then, the minimum brightness is divided by the maximum brightness. The resultant represents the brightness uniformity. The closer to one the resultant is, the smaller the difference between the minimum brightness and the maximum brightness is, that is, the more uniform the brightness distribution is.
However, since the amount of light that reaches regions far from the light source 51 is small, the amount of light reflected by the light admission planes 57a in the region is small. Accordingly, only a small amount of light exits through the exit plane 53 in the regions far from the light source 51. This results in an uneven brightness. In other words, the brightness distribution is uneven. To improve the brightness uniformity, wedge plate type light guiding plates have been proposed (for example, Japanese Laid-Open Patent Publications No. 11-242220 and No. 2000-21224).
A wedge plate type light guiding plate refers to a light guiding plate in which the thickness d1 of the light guiding plate 52 at the incidence plane 54 facing the light source 51 is greater than the thickness d2 of the light guiding plate 52 at the end opposite to the incidence plane 54 (d1>d2) Therefore, the reflection-exit plane 55 is inclined such that the distance between the reflection-exit plane 55 and the exit plane 53 decreases. Compared to flat plate types, grooves 57 far from the light source 51 receives and reflects a larger amount of light from the light source 51 in a wedge plate type light guiding plate. This improves the brightness uniformity.
However, if the thickness d2 of the wedge type waveguide 52 is further reduced to improve the brightness uniformity, the difference between the thickness d1 and d2 is increased. As a result, the inclination angle of the reflection-exit plans 55 is increased. This creates parallax and degrades the visibility.
Another method for improving the brightness uniformity is to vary the pitch or the grooves 57. However, if the pitch does not match the pixel pitch, interference patterns will appear.